In the United States, electric power is generated so as to provide an AC voltage in a desired voltage range and at a desired frequency, 60 Hz. Such power is generated by anyone of a number of power generation plants or power generation methods including for example large fixed electrical power generation facilities using fossil fuel (e.g., natural gas, coal, or fuel oil) or nuclear energy to generate electricity, hydroelectric power generation facilities, and pumped hydroelectric power generation facilities. The particular mixture of electrical power generation facilities that are to be operated at any given time are selected based on cost and other factors including regulating the grid voltage as to be within a desired range and regulating the frequency of the AC voltage so as to be at or about the desired frequency.
There is shown in FIG. 1A, an exemplary daily load curve that illustrates the variation as a function of time of the electrical load that is created by users and imposed on the electrical grid or electrical generation and distribution system that interconnects the power generation facilities and the various electrical load sources. Such electrical load sources include households, businesses, manufacturing facilities, computer facilities, and transportation services (e.g., mass transit systems powered by electricity). Although the curve illustrated in FIG. 1A suggests that the load changes occur over a large time period, in actuality load changes are extremely rapid (e.g., subsecond) as the various electrical power users or consumers adjust their individual electrical power requirements. There is shown in FIG. 1B an exemplary load curve 5 that illustrates the short-term variation in the daily load illustrated in FIG. 1A between the hours of midnight and 3 AM.
Because of such short-term variations that occur in the electrical load imposed on the electrical generation and distribution system and because the ability of a generator to follow such load variations is typically much slower than the time period of the load variation, the electrical generation and distribution system is constantly challenged with a mismatch between the load and the power being generated. As is known in the art, when the electrical load exceeds the total power being generated, the system the AC frequency drops. Alternatively, when the total power being generated exceeds the electrical load requirement, the frequency rises.
There is shown in FIG. 2, an exemplary curve that illustrates the variation in AC frequency as a function of time over a 24 hour time period. As illustrated, the mean AC frequency is 60.002 Hz, the maximum AC frequency is 60.063 Hz, and the minimum AC frequency is 59.944 Hz. ill a number of applications, such changes in AC frequency cannot be tolerated and so the power producer or consumer must install a system that is interconnected to the input from the electrical output generation and distribution system and that locally regulates the AC frequency so as to be within a desired range or at a desired value.
Consequently, the power output of the electrical generation equipment is being constantly adjusted so as to match the total power being consumed by or lost to the transmission and distribution and consumed by the customer load(s). In order to keep both the voltage and AC frequency within desired limits and/or ranges, the difference between the load and the power being generated is determined periodically and this difference is used to increase or decrease the output of the generators/power generation facilities. In this regard, and as known to those skilled in the art, real power generation/absorption is associated with frequency regulation and reactive power generation/absorption is associated with voltage regulation.
More particularly, a regional dispatching location or center is tasked with monitoring load and power generation as well as other factors (e.g., correction for frequency shift or time error) and outputs a control/dispatch signal (e.g., sometimes referred to as an energy management system (EMS) signal) to one or more generators/power generation facilities to increase or decrease the power output of such generators/power generation facilities. According to one technique, a large number of monitoring points or nodes are defined for a given area and the dispatching location or center monitors each of the monitoring points as to the foregoing factors. In addition and as described further herein, such a determination also can result in additional generating capacity being brought on-line so as to increase the total generating capacity of the power generation and distribution system or to take on-line generating capacity off-line, or put it in a standby on-line condition, so as to decrease the total generating capacity of the power generation and distribution system.
For example, a determination is made of the difference between the load and power generation, and a control/correction signal is outputted periodically (e.g., every 2-4 seconds) to the power generator/power generation facility(s). Because of the time lag associated with increasing or decreasing power by a given power generator, however, the effect on power generation by the signal being in outputted is delayed in time. There is shown in FIGS. 3A,B, generation and load curves that illustrate the responsiveness of a fossil-fuel power generator (FIG. 3A) and a pumped Hydro power generator (FIG. 3B) to a varying load.
As implemented in most electric power grids, regulation is a function or parameter that involves the use of on-line generation that is equipped with automatic generation control (AGC) and that can change output quickly (MW/minute) to track the moment-to-moment fluctuations in customer loads and to correct for the unintended fluctuations in generation. In so doing, regulation helps to maintain interconnection frequency, manage differences between actual and scheduled power flows between control areas, and match generation to load within the control area. Regulation requires faster response than can be obtained from units responding to market signals alone. Generators offer capacity that can be controlled by the system operator's AGC system to balance the power system.
One methodology for determining the amount of regulatory power demand uses an area control error (ACE) score. Such a method typically determines the ACE by comparing the power being delivered and the power required. In addition, the ACE being generated can also take into consideration corrections for frequency shift and/or other minor corrections such as a time error and a manual adjustment. In this method, as the ACE value rises or lowers, the regulation power being provided is adjusted accordingly. A low ACE score typically means that the power generation in distribution system is close to optimal operation from the standpoint of AC frequency. There is shown in FIG. 3C, an illustration of an algorithm of one technique for determining an ACE.
Control areas are not able and not required to perfectly match generation and load. The National Electricity Reliability Council (NERC) has established the Control Performance Standard (CPS) to determine the amount of imbalance that is permissible for reliability purposes. CPS 1 measures the relationship between the control area's area control error (ACE) and the interconnection frequency on a 1-minute average basis. NERC requires control areas to restore the generation/load balance within 15 minutes. Reserve service definitions require full reserve response within 10 minutes. The additional 5 minutes is provided for the system operator to assess the situation and respond.
CPS1 values can be either “good” or “bad.” When frequency is above its reference value, under-generation benefits the interconnection by lowering frequency and leads to a good CPS 1 value. Over-generation at such times, however, would further increase frequency and lead to a bad CPS1 value. CPS1, although recorded every minute, is evaluated and reported on an annual basis. NERC sets minimum CPS1 requirements that each control area must exceed each year.
CPS2, a monthly performance standard, sets control-area-specific limits on the maximum average ACE for every 10-minute period. Control areas are permitted to exceed the CPS2 limit no more than 10% of the time. This 90% requirement means that a control area can have no more than 14.4 CPS2 violations per day, on average, during any month.
Typically, approximately one percent (1%) of the power being transmitted and/or capable of being generated and delivered to the utility grid or electrical distribution system is reserved to fine tune or regulate the AC frequency of the electrical power being distributed. Such a source of electrical power is typically maintained in an on-line status with respect to the generation and distribution system, so as to be capable of supplying electric power at full capacity typically within at least a predetermined time period of a request for such power production. In present day applications the predetermined time period is typically about five minutes. Also, present regulatory standards for the generation of electrical power (e.g., standards set by FERC or NERC) require that the distribution system be balanced at least once every ten minutes and that over time the over and under generation nets to zero (0). As a result the correction typically can command a generator to alternatively load and then un-load hundreds of time a day. Although such a mode of operation is achievable with some types of power generating facilities, such a mode of operation has a negative impact on the life and overall deficiency of a typical fossil-fueled electrical generator (e.g., gas turbine powered electric generator).
In some cases, battery farms have been established and configured so as to have electrical capacities on the order of 500 kW to about 40 MW. While batteries are used to form an un-interruptible power supply, cyclical operation of a battery such as that undergone when performing fine tune regulation of AC frequency can prove to be damaging to the battery, including affecting operational life as well as the battery storage capacity.
In addition, the power dispatcher also can use the determined power difference to determine if supplemental or replacement reserve capacities should be brought online to increase the power generation capability of the power generation and distribution system. For major deviations from the desired or nominal AC frequency (e.g. 60 Hz), the dispatcher would output a signal and/or a request to bring spinning reserves that are typically on-line up to power and/or a signal/request to bring supplemental, and/or replacement reserve capacity on-line. Such reserves are typically capable of providing larger blocks of power as a complement to the power sources utilized for fine tuning of the AC frequency, however, such reserves typically take a longer time to reach full capacity (e.g., on the order of 10-15 or 30-60 minutes) and thus are not practically speaking useable for the fine tune regulation of AC frequency.
It thus would be desirable to provide new methods, systems and apparatuses for regulating AC frequency of the electrical power being generated. It would be particularly desirable to provide such methods, systems and apparatuses that can regulate AC frequency with a faster response as compared to conventional/prior art methods, systems and apparatuses and in a environment where load changes are highly variable or cyclical. It also would be particularly desirable to provide such methods, systems and apparatuses that embody one or more flywheel energy storage systems as the mechanism for regulating AC frequency. It also would be particularly desirable to provide such methods, systems and apparatuses that embody one or more flywheel energy storage systems in combination with a switchable load as a mechanism for regulating AC frequency. It also would be particularly desirable for such methods, systems and apparatuses to be adapted for use with existing methodologies, systems and apparatuses for determining and/or correcting for the difference between power generation and load. It also would be particularly desirable for such systems and apparatuses to be easily integrated or adapted for use with existing electrical power distribution systems.